The invention relates to a ram boring implement having a pneumatically or hydraulically driven percussion piston, movable in a reciprocating manner in a housing in the axial direction, and an axially movable bit which is arranged on the front end of the housing and is acted upon directly or indirectly by the percussion piston.
Ram boring implements of this type are used for driving earth bores for cables and the like to be laid underground. The bit is used to shatter and displace the earth. The movable arrangement of the bit offers the advantage that the percussion energy of the percussion piston can first be specifically transferred to the bit so that greater shattering energy is available.
German Patent Specification No. 2,157,259 discloses a ram boring implement of the above-mentioned type, in which the bit acted upon directly by the percussion piston or by an intermediate piston is pretensioned elastically into its rear end position by means of a spring supported on the housing. This has the disadvantage that a portion of the percussion energy is absorbed by the restoring spring during each blow. Moreover, if no resistance is offered to the bit, this results in the disadvantage that the bit constantly executes idle blows during which it reaches very high speeds which may result in damage to the implement.
German Patent Specification No. 3,124,524 describes a ram boring implement in which the bit is elastically pretensioned into a front end position in which it is not struck by the percussion piston. If no resistance or only a slight resistance is offered to the bit, the percussion energy is therefore directly transferred to the housing, and only if the bit strikes an obstacle and is thrust back into the housing is it struck by the percussion piston so that the obstacle is shattered. In this arrangement, although damaging idle blows are avoided, a portion of the percussion energy is used up here too by the spring, since the housing has to be accelerated forward against the spring force when the bit located in the front end position strikes an obstacle. Moreover, in loose soil, where the housing is only slightly supported in the earth, there is the risk of the housing being thrust back by the bit, supported at the front, and the pretensioned spring.
German Utility Model No. 8,310,178 discloses a ram boring implement in which the bit is mounted in a freely movable manner in the front part of the housing and is separated from the percussion piston by a housing wall. Here, the percussion energy is transferred to the bit by an impulse blow imparted by the housing. However, this has the disadvantage that a portion of the percussion energy is inelastically absorbed by the housing during the impulse blow if the bit does not bear against the housing wall during the initial phase of the blow. In stony soil formations, therefore, there is often not enough shattering energy available.